What term describes a substance that does not conduct heat or electricity?

The term you’re looking for is insulator. An insulator is a substance that inhibits the free flow of electrons, making it poor at conducting heat or electricity. In terms of thermal conductivity, insulators do not allow heat to pass through them easily; this property is utilized in various applications, such as in building materials to maintain temperature or in electrical components to minimize energy loss.

In contrast, conductors are materials that allow for significant movement of electrons, facilitating the transfer of heat and electricity. Reactors pertain to devices or systems designed to facilitate chemical reactions, and transmitters are typically associated with devices that send signals or data but do not describe the conductive properties of materials. Hence, insulators stand out as the correct term for substances that do not conduct heat or electricity.